An optical fiber consists of two basic components namely the core and the cladding. In a typical optical fiber, the optical signal is transmitted through the core of the fiber which has a higher index of refraction than the cladding. The greater index of refraction in the core provides continuous reflection of the optical signal into the core (i.e. a total internal reflection), thus minimizing losses due to refraction into the cladding. An additional buffer layer can also be used to provide protection for the cladding and the core. The buffer layer adds additional mechanical strength to the fiber to prevent cracking and breaking.
There are two different kinds of optical fibers, i.e., the multi-mode fiber and the single-mode fiber. Multi-mode fibers have a relatively large core diameter and provide several different paths for the optical signal to travel. The single-mode fiber is smaller in diameter and allows only a single path for the optical signal.
Multi-mode fibers are available in two different types of core structures or index profiles, a step-index and a graded-index. In a step-index multi-mode fiber, the core is made of a material with a uniform index of refraction, and the cladding material has a different index of refraction. This creates an abrupt interface between the core and the cladding. The advantage of this type of fiber is its ease of connection and splicing due to a large core size where alignment is not critical. A disadvantage of this fiber is that it suffers modal dispersion.
The core of a graded-index multi-mode fiber has a refractive index that gradually decreases toward the outer areas of the core. As light travels to the outer areas of the core it moves faster due to the lower index of refraction. This causes all modes to arrive at the output at the same time, reducing modal dispersion. A graded-index multi-mode fiber is simple to splice and connect due to its larger core diameter.
Single-mode fibers are normally available with a step-index profile, even though some dispersion-shifted fibers are available but are not often used. Since there is only one path through the core, there is no modal dispersion. Single-mode fibers are used for high speed applications and where long distance transmission is required. A single-mode fiber system is a highly reliable one because it utilizes fewer components, such as repeaters, than the multi-mode fibers. The disadvantage of single-mode fibers is the difficulty in connection and splicing due to the extremely small core diameter.
In a step-index fiber, the refractive index of the core and the cladding of the fiber changes drastically at the interface between the core and the cladding. In an optical fiber that has many mode of optical transmission, each mode proceeds at a different speed. There is a serious loss of signal which restricts the frequency bandwidth in an optical fiber that has many modes of optical transmissions. In order to correct this problem, optical fibers having only a single mode of optical transmission are used. The core diameter of a single-mode optical fiber is in the dimension of microns and, therefore, allows only one bundle of light into the core. The light arrives at the end of the fiber at the same time since any possible scattering of a multi-mode fiber is eliminated. A single mode optical fiber also transmits light at a wider bandwidth. However, due to the small core, it is difficult to splice fibers together. It is therefore desirable to provide optical fibers that have gradually changing refractive indices.
Optical fibers that have a continuous distribution of refractive indices from the cladding to the core have been made by others with glass or quartz. However, these types of optical fibers are produced at a very low production rate and very high cost by ion-exchange or sol-gel methods. Optical fibers of glass or quartz have the problem of poor flexibility and processability for use in many applications. These types of optical fibers are disclosed in Japanese patent publication 47-816.
Others have proposed methods of making optical fibers that have a continuous distribution of refractive index from the cladding layer to the core of the optical fiber from polymeric materials. One such method is the manufacturing of a synthetic resin filament by an ionic grafting polymerization technique in which the concentration of metal ions is made to continuously change from the core of the fiber to the cladding layer. A polymeric optical fiber that has a continuous distribution of refractive index from the cladding to the core is thus produced. Such a method is described in Japanese patent publication 47-26913.
Another method proposed the making of a fiber from a mixture of two or more clear polymeric resin having different refractive indices. After a special solvents treatment, part of the mixture of the resin is dissolved to obtain an optical fiber. This technique is disclosed in a Japanese patent publication 47-28059.
Another method, disclosed in Japanese patent publication 54-30301, teaches a polymerization method in which two polymers having different refractive indices are used to form a polymer that has a continuous refractive index distribution from its surface layer to the center of the fiber.
Still another method disclosed a diffusion method of a monomer into the surface layer of a block copolymer such that the monomer content in the block copolymer forms a continuous distribution from its surface layer to the center of the block copolymer. A polymerization reaction is then performed to make a polymeric optical fiber that has a continuous refractive index distribution. This technique is disclosed in Japanese patent publications 52-5857, 56-37521 and 57-29682. It should be noted that all of the above methods and techniques disclosed are performed in a non-continuous, batch-type operation.
In order to overcome the deficiency of a non-continuous production process, continuous production methods were proposed by Japanese patent publication 1-1896021, 1-253704, 2-16505 and 2-233104. In these techniques, a polymer and a monomer are mixed in a mixing tank and heated until the polymer is dissolved in the monomer and uniformly mixed. A monofilament is then extruded from a die and fed into a gas evaporator. The gas is blown into the evaporator such that the monomer evaporates from the surface of the optical fiber. An optical fiber having a continuous concentration distribution of the monomer is thus formed. After a hardening process, a polymeric monofilament having a continuous refractive index distribution from its surface layer to its center is obtained.
The methods described above have several shortcomings such as a long diffusion time required which leads to excessive manufacturing time and low production rate, the difficulties in selecting optimal production conditions and the poor reproduceability. These problems must be solved before the methods can be used in a production environment.
It is therefore an object of the present invention to provide a method of manufacturing optical fibers of polymeric materials that have a continuous distribution of refractive indices from its cladding layer to the core of the fiber that does not have the shortcomings of the prior art methods.
It is another object of the present invention to provide a method of manufacturing optical fibers of polymeric materials that have a continuous distribution of refractive indices in the fiber that can be used in a continuous manufacturing process.
It is a further object of the present invention to provide a method of making optical fibers of polymeric materials that have a continuous distribution of refractive indices from the cladding layer to the core of the fiber that can be produced in a production process with a high degree of reproduceability.